1. Field of the Invention
The present invention relates generally to 2-wavelength image sensors and particularly to 2-wavelength image sensors picking up both visible and infrared images.
2. Description of the Background Art
An optical sensor system utilized in the field of security for detecting human bodies can be improved in performance for detection by combining picking up a visible image and picking up an infrared image. One such sensor system conventionally used is a sensor system that simply combines the two pickup systems of a camera for visible radiation and an infrared camera. However, such a system is large in size and requires a complicated image process between two types of optical images to analyze data. Accordingly there has been a demand for a sensor system capable of picking up both types of images with a single camera.
In connection therewith there has been proposed a 2-wavelength image sensor capable of picking up a visible image and a thermal image with a single chip. For example, Japanese Patent Laying-Open No. 11-289492 discloses an infrared image sensor employing a PtSi Schottky diode as a detector with a PtSi layer provided in a thin film to also have a function to detect visible radiation. An optical sensor system employing this image sensor has a spectroscopic filter exchanged to employ picking up a visible image and picking up an infrared image, as required.
Furthermore a typical color image sensor employing a Si photodiode in a photodetector is capable of picking up a near infrared image, which is not a thermal infrared image, and a visible image. On the photodiode a spectroscopic filter is provided to allow an image for light different in wavelength, such as blue, green, red, near infrared, to be picked up with a single chip. For example, Japanese Patent Laying-Open No. 2005-051791 discloses a color image sensor picking up visible to near infrared images that defines how a color (i.e., a spectroscopic filter provided on a detector) is arranged to allow a near infrared image to be higher in resolution than a visible image.
Generally for an optical sensor system utilizing both visible and thermal infrared images the resolution required by one image is different from that required by the other. More specifically, for the visible image, it is desirable to provide an increased resolution to pick up the image in detail, whereas for the thermal image, while resolution may somewhat be low, high sensitivity (or temperature resolution) is desired.
The image sensor disclosed in Japanese Patent Laying-open No. 11-289492, however, detects visible radiation and infrared radiation with a single detector and it is inevitable that the visible image and the thermal image are disadvantageously the same in resolution. Furthermore the single detector prevents picking up the visible image and the thermal infrared image simultaneously. Furthermore the detector must be cooled to approximately −200° C. or lower. As a result the system is increased in size and cost for production.
The image sensor disclosed in Japanese Patent Laying-open No. 2005-051791 allows two types of images to be different in resolution, i.e., can increase the resolution of the near infrared image to be higher than that the visible image. However, the image sensor has near infrared pixels higher in resolution distributed thinned out in arrangement. Accordingly it requires pixel interpolation to obtain a complete image.